Results of in vivo studies indicate that the calorigenic effect of thyroid hormone is expressed via activity of the cellular 'sodium pump' (Na ion, K ion-dependent adenosine triphosphatase; Na, K-ATPase). The mechanism of this effect is unknown because an in vitro model has not been available in which to study iodothyronine action on this enzyme system. We have developed a rat renal cortical tubule preparation (collagenase digest) in which Na, K-ATPase activity is highly and rapidly (3-8 hr) responsive to thyroxine (T4) (10 to the minus 12th power M) and triiodothyronine (T3). Homogenates of the digest are also responsive to iodothyronines. In this model system the action of T4 and T3 is independent of protein synthesis and thus extranuclear. We propose studies to define the mechanism of action of thyroid hormone in this model. Total ATPase is measured by Pi generation from added substrate (ATP) and the fraction inhibited by caubain (10 to the minus 3rd power M) is defined by oaubain (10 to the minus 3rd power M) is defined as Na, K-ATPase activity. Thyroid hormone effect is to be analyzed through studies of activities of iodothyronine analogues and altered concentrations of ions (Na ion, K ion, Mg2 ion, Ca2 ion) on Na, K-ATPase, through possible interactions of labeled iodothyronines with membrane protein(s) or with ATPase activators known to exist in cytoplasm. Correlations will be carried out of 86Rb ion transport, ATPase activities and O2 consumption (Warburg) by intact cells as functions of thyroid hormone concentrations (10 to the minus 8th power to 10 to the minus 14th power M). The relationship of the iodothyronine effect we describe to that of Lo and Edelman (in which new enzyme subunits are synthesized after 24-28 hours of T3 administration to intact rats) is to be studied through maintenance of our model for up to 48 hours. Variables will be levels of iodothyronines, length of exposure of cells to hormone and the presence or absence of inhibitors of protein synthesis.